There are two backends for Squirrelmail: the standard backend stores users'
profile/preferences in text files (file_prefs.php), and the MySQL one stores
them in a MySQL database (db_prefs.php).

This one adds an LDAP backend. However, things are treated
much differently, due to the difference in nature between LDAP and other
databases. Namely: LDAP writes are expensive. Extensive caching is used to
reduce the overhead on the LDAP server, especially in a setup with a lot of
users.

This makes a big difference, from the user's point of view. Whenever
changing some option, the change is for the current session only. The change
will only be saved permanently, if and only if the user logs out from
Squirrelmail, thus ending the session.